The present invention relates to a positive-working chemical-amplification photoresist composition or, more particularly, to a positive-working chemical-amplification photoresist composition capable of giving a very finely patterned resist layer on a substrate surface by patterning exposure using a KrF excimer laser beam.
As a trend in the manufacturing technology of semi-conductor devices and liquid crystal display panels in recent years, extensive investigations are now under way to establish a photolithographic patterning process of a resist layer having a pattern resolution of as fine as 0.25 μm or even finer by the use of a positive-working chemical-amplification photoresist composition. Turning now to the problem of the light source for the pattern-wise exposure of the photoresist layer to comply with the requirement in the manufacture of semiconductor devices to accomplish finer and finer patterning, a photolithographic patterning technology for obtaining a patterned resist layer of 0.15 to 0.22 μm fineness by using a KrF excimer laser beam is the current target of the development works.
With an object to comply with the above mentioned requirements, a proposal is made in Japanese Patent Kokai 7-209868 for a positive-working chemical-amplification photoresist composition containing, as the film-forming resinous ingredient, a copolymeric resin consisting of hydroxyl group-containing styrene units, styrene units and tert-butyl (meth)acrylate units in a molar ratio of 40:20:40 or 33:17:50. The there proposed photoresist composition using a copolymeric resin with a relatively small amount of the hydroxyl group-containing styrene units or a relatively large amount of the tert-butyl (meth)acrylate units is not quite satisfactory when an extremely fine patterned resist layer of 0.15 to 0.22 μm fineness is formed therewith because the cross sectional profile of the patterned resist layer is not fully orthogonal as desired.
In recent years, on the other hand, onium salts or, in particular, those containing sulfonate ions as the anion are highlighted as an acid-generating agent used in positive-working chemical-amplification photoresist compositions. Examples of such an onium salt as the acid-generating agent in a positive-working chemical-amplification photoresist composition heretofore proposed include salts formed from a bis(4-tert-butylphenyl) iodonium cation and an anion selected from the group consisting of camphor sulfonate ions, 4-methylbenzene sulfonate ions and trifluoromethane sulfonate ions as disclosed in Japanese Patent Kokai 9-179302 and onium salts containing a fluoroalkyl sulfonate ion and having at least one acid-instabilized group on a phenyl group in the molecule such as 2,2,2-trifluoroethane sulfonic acid (4-tert-butoxyphenyl) diphenyl sulfonium as disclosed in Japanese Patent Kokai 10-7650.
These acid-generating agents, however, are not quite satisfactory with various disadvantages. For example, the onium salts having a camphor sulfonate ion or 4-methylbenzene sulfonate ion as the anion have a relatively low solubility in the organic solvent for the photoresist compositions so that the photoresist composition cannot be compounded with a sufficiently large amount of the onium salt not to impart the photoresist with high photosensitivity. In addition, the stability of the photoresist solution containing the same cannot be high enough and, due to the large molecular size of the compound, the distance of diffusion of the molecules in the post-exposure baking treatment is necessarily not so large affecting the resolution in patterning. On the other hand, the molecular size of the trifluoromethane sulfonate ions is too small with an excessively large distance of diffusion to cause a defect in patterning such as thinning of the resist pattern.
Further, the onium salts having an acid-instabilizable group and containing a fluoroalkyl sulfonate ion in the molecule are defective due to their low solubility in the organic solvent of the photoresist solutions to limit the amount thereof in the photoresist solution resulting in a decreased photosensitivity and low storage stability of the photoresist solution. In addition, the acid-instabilizable group is liable to be decomposed when the pre-exposure baking treatment or, namely, heat-drying of the photoresist layer formed by coating is performed at 120° C. or higher eventually to lose the activity as an acid-generating agent.
Besides the above, it is known that bis(4-tert-butylphenyl) iodonium nonafluorobutane sulfonate can be used as the acid-generating agent in a photoresist composition for exposure with an ArF excimer laser beam of 193 nm wavelength. It is not certain at all, however, whether or not good results can be obtained when the above mentioned acid-generating agent is used in a photoresist composition for exposure with a KrF excimer laser beam because of the difference in the resinous base ingredient of the composition which is an acrylic resin or a copolymeric resin of a cyclic olefin monomer and maleic anhydride in the former photoresist composition and a copolymeric resin of styrene, α-methyl styrene and a (meth)acrylate ester monomer in the latter photoresist composition.